Preheat the gas grill to med. high or prepare a medium hot fire on a charcoal grill with a cover.
Place a medium size heavy fire proof pan half filled with water on the grill. Place the package of frozen spinach in the water to thaw for about 10 minutes.
Meanwhile to make the dressing in a medium sized bowl combine 1/2 cup sour cream, 2 teaspoons of chopped onion, chopped green apple, and 4 tablespoons of creamy poppyseed dressing. Put in the refrigerator until ready to use. Slice the tomatoes. Tear off 12 iceberg lettuce leaves to fit the buns. Then stem the spinach leaves. Put 1 cup of shredded parmesian cheese in a small bowl and set aside.
To make the slider patties:
As soon as the spinach is thawed, remove from the water. Open the package, measure out 8 ounces and put in a large bowl. Add the 1lb of ground round. Combine, handling as little as possible. Make 12 meatball sized portions and flatten to make patties. Put on a platter and set aside. Grill the bottom portion of the buns for 45 seconds on each side. Remove and place on a platter. Brush the grill with vegetable oil. Put the sliders on the grill, and add 1/8 teaspoon of garlic salt to each patty. Grill each side for 2-3 minutes. Remove and place on a platter when done.
To prepare the bun tops, put 1 1/2 teaspoons butter on the flat side of each bun top, then dip butter side down into the bowl of shredded parmesian cheese, pressing down slightly. Put the buns cheese-side down on the grill for about 45 seconds. Turn them over and grill them on the other side for 45 seconds. Remove them to a platter.
To assemble the sliders:
Get the dressing from the refrigerator. To dress the bottom of the bun, place 1 tablespoon of the dressing on each of the bun bottoms. Then add a piece of iceberg lettuce and place a slider on each piece of lettuce. Add three spinach leaves and a tomato slice. Place the tops on each one and serve!